Optical communication systems are a substantial and fast growing constituent of communication networks. The expression “optical communication system,” as used herein, relates to any system that uses optical signals to convey information across an optical waveguiding medium, for example, an optical fiber. Such optical systems include but are not limited to telecommunication systems, cable television systems, and local area networks (LANs). Currently, the many optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each signal source, the complete signal being constructed from the portions of the signals collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of existing fiber optic networks. WDM systems typically include a plurality of transmitters, each respectively transmitting signals on a designated one of a plurality of channels or wavelengths. In a simple point-to-point network, the channels are combined by a multiplexer at one end terminal and transmitted on a single fiber to a demultiplexer at another end terminal where they are separated and supplied to respective receivers. In more complex systems, an add/drop multiplexer may be present at each node for dropping one or more particular channels from the DWDM signal, and subsequently adding the one or more channels back to the signal prior to transmission to another network node.
Even in DWDM systems, however, fiber capacity is being exceed, resulting in congestion of the data carried on many networks. One way to relieve the congestion is to leverage technology advances to transmit data at higher rates. To accomplish this, all the equipment on a network could be upgraded to the higher data rate. This is, however, quite expensive and time consuming. A preferable method of relieving congestion would be to increase capacity on particular spans without requiring an upgrade of all network equipment, particularly on spans where congestion is not yet problematic. This approach requires a method of collecting lower data rate tributary channels and assembling the channels into a higher data rate aggregate signal.
To date, the prior art has failed to provide an effective, reliable, and cost-efficient multiplexer/demultiplexer for serving this purpose. A main difficulty in this regard relates to maintenance of the Payload and transport overhead (TOH) bytes in the tributary signals. Once the lower data rate tributary signals are combined into the aggregate according to a standard SONET/SDH method, the overhead bytes in the tributary signals are irretrievably lost. This seriously limits the ability to provide line maintenance signaling, section/line/path performance monitoring, and fault isolation for the tributary signals.
Thus, there is a need for a semi-transparent time division multiplexer/demultiplexer, which transmits low rate tributaries from one location to another using a high rate aggregate connection, while preserving the TOH and Payload for each tributary.